This invention relates to traction control systems used on motor vehicles and which is an enhancement to brakes only traction control systems.
Brakes traction control systems (BTCS) are typically used when driving off-road. BTCS in its most basic form acts as an electronically controlled variable slip differential lock for a drive axle on a vehicle in which torque is applied to wheels at opposite end of the axle via a conventional differential unit. The BTCS monitors the rotational speed of the wheels via wheel speed sensors and each wheel is provided with an independent braking actuator. The information from the sensors is fed to a brake system controller where the information is processed and the brakes to the wheels operated with the aim of improving traction.
If the vehicle is being driven on a non-homogenous surface and sufficient driving torque is applied it is possible for a wheel having a lower surface adhesion to loose grip and spin. In this situation, this wheel will require a relatively low torque, as compared with the other wheel, to cause it to xe2x80x9cover spinxe2x80x9d. If the axle has a conventional differential, then the differential will permit only this same low torque to be transmitted to the non-spinning wheel. This level of torque may not be sufficient to maintain vehicle movement. The BTCS causes the brake actuator associated with the spinning wheel to apply the associated brake and increase its torque output in order to transfer torque across the differential to the non-spinning wheel. The non-spinning wheel then gains sufficient torque to achieve vehicle movement and its speed of rotation increases. The BTCS controls both wheels until their rotational speeds equalise.
A problem with this type of BTCS is that if the vehicle does not accelerate quickly, the brakes when applied to the spinning wheel may cause engine stall. Some BTCS are programmed to recognise engine stall and implement strategies to prevent this happening. This may leave the vehicle immobilised with the engine held in low revs and unable to supply the necessary torque to move the vehicle. Such a situation may be experienced with vehicles having small engines when driven off-road.
According to the present invention there is provided a vehicle brakes traction control system for a vehicle having at least one drive axle with a differential operable between wheels at the two ends of the axle, and brakes for each respective wheel operable by a brake controller, the traction control system including wheel speed sensors which sense the rotational speed of said wheels, an engine torque demand sensor and an engine torque output monitor, the brake system controller receiving signals from said sensors and monitor and being operable to apply the brakes to equalise the rotational speed of said wheels when the engine torque output is less than a desired torque output for a given torque demand.
The torque demand sensor may be a throttle opening sensor such as an accelerator pedal position sensor, and the torque output monitor may include a torque output map.
The engine speed at which maximum torque output is targeted may be altered in relation to the rate of change of torque demand in particular so that the rate of torque build up at the wheels is proportional to the rate of change of torque demand. When the control system is for use in a vehicle having an engine management system (EMS) with an engine map stored therein which includes information relating to the engine speed at which the maximum torque is available for a given torque demand e.g. throttle opening, the brake system controller operates the vehicle brakes only when excess rotational speed is detected for one of said wheels and when in receipt of a signal from the EMS indicating that the engine speed is above that required for maximum torque output. Such a system is self tuning for any EMS.
Alternatively, the brakes controller has an engine map stored therein which includes information relating to the engine speed at which the maximum torque is available for given throttle openings, wherein the brake system controller operates the vehicle brakes when excess rotational speed is detected for one of said wheels and when the engine speed is above that required for maximum torque output.
The engine torque output may be measured as a function of rotational speed of the engine.
Also according to the present invention there is provided a method of operation of a vehicle brakes traction control system for a vehicle having at least one drive axle with a differential operable between wheels at the two ends of the axle, wherein the system is operable to apply the vehicle brakes to equalise the rotational speed of said wheels only when the engine torque output is less than a desired torque output for a given engine torque demand.